1. Field of the Invention
This invention relates to low stress, low residual reflection multi-layer anti-reflection coatings for optical lenses and, in particular, to a composition for forming a high refractive index anti-reflection coating and a composition for forming a low refractive index anti-reflection coating, methods for making optical lenses preferably using the composition including using a conventional vacuum deposition chamber employing an optical monitor to control the optical properties of the anti-reflection coating.
2. Description of Related Art
It is well known in the optical arts that a reflection of light off glass and other surfaces is undesirable or creates visual sight discomfort. The reflected light makes a user feel dizzy or causes an image to be blurred, among other such undesirable effects. For optical lenses this is of particular concern and compositions and methods have been developed for reducing the reflection of light off the optical lens surface.
A considerable number of anti-reflection (AR) coatings have been suggested in the prior art for a primary design purpose of ensuring that the residual reflection will be held to a relatively small value over the entire range of the visual spectrum. Single or double layer coatings have provided significant improvement but the residual reflections are still more than desired and to improve the AR properties, the prior art has resorted to AR coatings having three or more layers.
The optical thickness of each deposited AR layer is typically controlled to optimize or maximize the AR effect and as well known the optical thickness of a layer is the product of the real (geometrical) thickness and the refractive index of the respective layer. The optical thickness is generally described in fractions of a wavelength of a designated reference light ray for which the coating is to be used. Frequently, the design wavelength will be about 510 nanometers (nm) to 550 nm. The optical thickness of respective AR layers may be defined by the following general formula where N is the refractive index, d is the geometrical thickness of the layer and λ is the reference wavelength:Nada=xλwherein x is a number, typically a fraction, indicating the fraction of the wavelength and a is an integer representing the layer coated with the lowest number being closer to the eyeglass lens. Typically x will be 0.25 which represents a quarter wavelength optical thickness.
As well known in the art today, the optical thickness of the individual layers can be adjusted to obtain the same results on substrates of different refractive indices.
In the formation of each AR layer, the deposited layer exhibits a maximum value of interference for every one fourth of the wavelength of light for measurement of the thickness, i.e., λ/4. Thus, the thickness of an optical AR layer is conventionally controlled during the formation thereof by utilizing this phenomenon with the optical thickness being multiples of 0.25.
While the following description will be directed to polycarbonate lens for convenience, it will be understood to those skilled in the art that the invention applies to other lens materials such as polyurethane, acrylic glass, CR-39, etc. Stress in a polycarbonate lens causes birefringence and optical distortion. While not visible under normal circumstances it is evident when polycarbonate is placed between two polarized filaments and this is one of the reasons that polycarbonate lenses are optically inferior to lenses such as glass, CR 39 and other such materials. The new polycarbonate lens developed by Optima, trade name Resolution®, is free of this stress and birefringence and thus the current processing to provide AR coatings and its inherent stress now becomes more of a concern to makers of such lenses.
In addition, the current state of AR coatings have a residual green reflection which varies between 0.75% and 1.5% residual reflection. This green color is cosmetically unpleasant and acts as a green filter which decreases the amount of green light the human eye perceives. A lower residual reflection with no filtering effect is much more desirable both in the performance of the coating and in its cosmetic appearance. It is preferred that only white light be reflected.
The current design and production of AR coatings are well understood in the industry today and typically the residual color is left in the design to make manufacturing much simpler and cheaper. Current technology uses a Quartz Crystal Monitor to control the physical thickness of the individual layer required to produce an AR coating. The current coating standards call for a 4-layer HLHL coating, where H represents a high index dielectric material chosen for its specific refractive index, and L represents a low index dielectric material also chosen for its refractive index. Each layer typically consists of an optical quarter wave of the high or low index material chosen. Low index materials include SiO2 and MgF2. High index materials include oxide sub groups of the following materials: Zr, Hf, Ta, Ti, Sb, Y, Ce, and Yb. While not inclusive, these materials are the most widely used today.
Many AR coatings being produced today also include an adhesion layer, a buffer layer, an abrasion resistance layer, and a hydrophobic outer layer. These layers are used to enhance the performance of the coating from a consumer standpoint but have very little effect on the optical qualities of the AR coating.
Another concern in the making of AR coatings is that the high index and low index material induce both compressive as well as tensile stress in the AR coating film. The current art of anti-reflection (AR) coatings, however, does not take into account the amount of stress inherent in the coating itself. This is because the current lens produced on the market today such as the polycarbonate lens has so much stress already that the additional amount of stress caused by the AR coating is not considered important. This is one of the reasons that current production techniques try to limit the number of layers used. In general, a low index material such as silica produces a tensile stress which is about 5 times the compressive stress produced by a high index material. If the coating becomes too thick with additional layers, the differences in stress caused by the low index material and high index material can cause the AR film to separate and come off the lens and also cause adverse optical effects.
Another reason current technology limits the number of layers is that the quartz crystal monitor is only capable of measuring the physical thickness of the applied materials. An AR coating however is designed around optical qualities, which are very dependent on the refractive indexes of the materials being used. These indexes will shift as coating conditions such as available O2, coating rate and deposition temperature change. The green reflectance left in the coating does an excellent job of hiding these imperfections during normal production and the high peak reflectance in the very broad green visible spectrum can shift during production and be unnoticeable to all except a well trained professional.
In order to form an AR coating with no residual color, i.e. white, and a low overall residual reflection, the manufacturer must typically add several additional AR layers. The added thickness created by these layers causes an increase in stress and possible AR coating delamination and these competing problems must be addressed by the lens manufacturer.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a composition for making a high index of refraction AR coating on an optical lens or other optical article.
It is another object of the present invention to provide a composition for making a low index of refraction AR coating on an optical lens or other optical article.
It is yet another object of the present invention to provide a method for making optical lenses or other optical articles having an AR coating using the above compositions.
It is still yet another object of the present invention to provide a method for making optical lenses having an AR coating using an optical monitor to provide a desired AR optical coating on the optical lens or other optical articles.
A further object of the present invention is to provide a method for coating optical lenses and other optical articles with an AR coating which has low residual reflection, the reflective light is essentially white light and the AR coating has low stress.
A further object of the present invention is to provide optical lenses and other optical articles made using the methods of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.